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Assessment of scatterometry as a new CD metrology technique

Our goal is to investigate the capability and limitations of existing metrology techniques towards 157 nm and to suggest alternatives when needed. ST Crolles has been especially active in the evaluation of a new metrology technique that was proposed by suppliers during the period covered by the project.

This new technique, called scatterometry, is supposed to enhance process control in microelectronics fabrication. This alternative solution was assessed during the project for linewidth or Critical Dimension (CD) control at 65nm node, with particular care to 5 issues:
- Timing and availability of suitable tools;
- Ultimate resolution;
- Accuracy and statistical precision;
- Interaction with materials;
- Universality.

The Scatterometry technique needs to be fully assessed before being introduced in a productive environment. Moreover, integrated metrology tools become available with a very good level of confidence, using scatterometry technique. For 157nm lithography, integrated scatterometry metrology was considered as an interesting and valuable solution. This solution was considered in the final assessment and recommendations for metrology.

Scatterometry is based on existing optical hardware tools, as spectroscopic ellipsometry for example. The method uses a specific pattern present on the wafer to be controlled, which is a 50 microns diffraction grating. The scatterometry tool measures the reflected light from this grating. The recorded data is then analysed with a software tool, based on the inverse resolution of light propagation. A complex software is developed and associated to the optical hardware tool, including a precise optical simulation tool and an efficient multi-parameter regression. Due to the heavy computation that are needed, real time operation is not possible at early phase. A library of possible solutions is simulated and the regression tool find out the best fit by scanning this pre-computed library.

The work towards scatterometry assessment has been done in several steps: information and review of announced performances; demonstration by several suppliers; on-site evaluation and application development; integration of the tool in production line.

1) Suppliers have disclosed a lot of encouraging information on scatteromettry performances and perspectives even beyond 65nm node.

2) Demonstration has been carried out on six different solutions. Wafers were prepared using e-beam tool or 193nm lithography scanners. High performances were generally obtained on current and advanced technologies (130nm-65nm node).

3) A spectroscopic ellipsometer tool was installed to provide scatterometry measurement. The hardware was upgraded and a library generation tool was set up. Different lots were tracked and checked. Experiments were carried out so as to verify technical capability of the tool on specific targets:
- Functionality tests and tool integration;
- Standard applications on critical layers (R&D);
- Advanced applications, i.e. line profile control, litho tools qualification.

Correlation with CD-SEM and cross-section SEM were performed. The values obtained with scatterometry present a constant offset with respect to CD-SEM standard measurement. Results are in general well correlated with cross section SEM pictures, with a much lower shift. Tool to tool matching has also been checked.

Measurement reliability is reported below:
- Repeatability < 0.2nm;
- Reproducibility < 0.4nm.

4) Integration activity consists in 65 nm node related work, mainly tools follow-up and validation for volume litho production. The scatterometry tool has been used to help process follow-up with very fast and efficient result. Tool qualification is also a very efficient application. Litho tool suppliers are also developing scatterometry-based qualification procedures for acceptance tests.

To garanty robustness of scatterometry tool, the impact of different defects has been evaluated. For example: defects in the scatteromettry target after lithography (like scratch defect, missing lines, pattern collapse or bridged lines), target size limitation towards future available area on the wafer and alignment on the scatterometry target. The results obtained do not show any show-stopper towards fab integration. According to fab quality criteria, results were very good after lithography and after etch.

Although some upgrades are needed, the CD-SEM is considered to be the primary tool class for the 157nm lithography. Several factors contribute to this evaluation: great expertise worldwide, good reliability and easy integration in production. But scatterometry is confirmed to be a very attractive technique, especially efficient for various applications in microelectronics fab and in supplier tool manufacture. Scatterometry tools are now operated in fabs, with high precision and fast throughput. Productivity gain have been showed.

More information on the UV2LITHO project can be found at:

Reported by

STmicroelectronics Sa
850 rue Jean Monnet
38926 Crolles
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